Recalibration of the Local Magnitude (ML) Scale for Earthquakes in the Yellowstone Volcanic Region

ABSTRACT The Yellowstone volcanic region is one of the most seismically active areas in the western United States. Assigning magnitudes (M) to Yellowstone earthquakes is a critical component of monitoring this geologically dynamic zone. The University of Utah Seismograph Stations (UUSS) has assigned M to 46,767 earthquakes in Yellowstone that occurred between 1 January 1984 and 31 December 2020. Here, we recalibrate the local magnitude (ML) distance and station corrections for the Yellowstone volcanic region. This revision takes advantage of the large catalog of earthquakes and an increase in broadband stations installed by the UUSS since the last ML update in 2007. Using a nonparametric method, we invert 7728 high-quality, analyst-reviewed amplitude measurements from 1383 spatially distributed earthquakes for 39 distance corrections and 20 station corrections. The inversion is constrained with four moment magnitude (Mw) values determined from time-domain inversion of regional-distance broadband waveforms by the UUSS. Overall, the new distance corrections indicate relatively high attenuation of amplitudes with distance. The distance corrections decrease with hypocentral distance from 3 km to a local minimum at 80 km, rise to a broad peak at 110 km, and then decrease again out to 180 km. The broad peak may result from superposition of direct arrivals with near-critical Moho reflections. Our ML inversion doubles the number of stations with ML corrections in and near the Yellowstone volcanic region. We estimate that the additional station corrections will nearly triple the number of Yellowstone earthquakes that can be assigned an ML. The new ML distance and station corrections will also reduce uncertainties in the mean MLs for Yellowstone earthquakes. The new MLs are ∼0.07 (±0.18) magnitude units smaller than the previous MLs and have better agreement with 12 Mws (3.15–4.49) determined by the UUSS and Saint Louis University.

SEISMICITY of AZERBAIJAN and ADJACENT TERRITORIES in 2015

In 2015, the network of Azerbaijan seismic stations included 35 digital stations, which continued to operate unchanged. The set of parameters determined for estimating the earthquake magnitude has decreased – determination of the MPVA magnitude and КР energy class has ceased. Only the local magnitude MLАзр was measured. The total number of earthquakes recorded by the Azerbaijan network of stations in 2015 amounted to 6419, however, only 128 of them with MLАзр3.0 are given in the catalogue of earthquakes of Azerbaijan published in the Appendix to this article. Seismic activity in the study area remained unchanged. The released seismic energy changed insignificantly compared to its value in 2014 and was close to the background level for the period 1993–2014. The increase in the number of earthquakes of the representative level (K≥8.6, MLАзр≥2.8) in comparison with the long-term average annual value is due to aftershocks of earthquakes on January 26 with MLАзр=4.9, June 3 with MLАзр=4.6, and September 4 with MLАзр=5.9. The 2015 most significant seismic event on the territory of the Republic was the earthquake on September 4 with MLАзр=5.9 and seismic intensity at the epicenter of I0=7.

Microseismicity of the Central Alpine Fault Region, New Zealand

<p>This study investigates the spatial and temporal patterns in microseismicity along the central section of the Alpine Fault, South Island, New Zealand. This section, between Harihari and Karangarua, has significantly lower seismicity than the regions to the northeast and southwest. Several hypotheses of mechanisms said to contribute to the anomaly have been proposed over the years including locked fault, slow slip, shallow creep and external fluids affecting the thermal regime and brittle-ductile transition. Focussing on the shallow crust, the contrasting seismic character is compared to the northern and southern sections from seismicity behaviour, focal mechanisms and seismogenic depth. A temporal array of eight seismographs (including three broadband instruments) was augmented with three GeoNet stations bounding the array. This provided an average spacing of 14 km and a magnitude cut-off of ML 1.6 compared to the GeoNet national network cut-off of ML 2.6 and station spacing of 80-100 km. The Gutenberg-Richter distribution for the four month time frame analysed defned a b-value of 0.75 plus or minus 0.06 which may indicate a locked, heterogeneous zone under high-stress from fluid pressure or a predominance of thrust mechanisms over the survey period. Seismicity over the deployment was within the average range of the last 15 years. The 'horseshoe' shaped seismicity pattern observed from long-term national catologue data is similar for smaller magnitudes. While the central portion of the Alpine Fault is quieter with unusually low b-value, the region is not aseismic. Neither does it experience the level of microseismicity seen in creeping faults. The brittle-ductile transition varies laterally along the fault and is estimated at up to 15 km for most of the survey region but closer to 10 km for the region associated with the highest orogenic uplift rates which compares well with past studies. A local magnitude scale was developed from direct linear inversion of the pseudoWood-Anderson amplitudes and event-station distances. A linear inversion of data from the standard New Zealand magnitude equation characterised an attenuation parameter of 0.0167 km minus 1; more than double the value used in national local magnitude calculations (of 0.0067 km minus 1). Swarm clustering dominates the seismicity character of the time frame. Utilising the earthquake relocation program HypoDD, a selection of clusters both near the Alpine Fault and away from it resolve to point sources. Those close to the Alpine Fault are located in what may be the footwall of the Fault which may indicate that the velocity model has located the events too far to the northwest.</p>

Microseismicity of the Central Alpine Fault Region, New Zealand

<p>This study investigates the spatial and temporal patterns in microseismicity along the central section of the Alpine Fault, South Island, New Zealand. This section, between Harihari and Karangarua, has significantly lower seismicity than the regions to the northeast and southwest. Several hypotheses of mechanisms said to contribute to the anomaly have been proposed over the years including locked fault, slow slip, shallow creep and external fluids affecting the thermal regime and brittle-ductile transition. Focussing on the shallow crust, the contrasting seismic character is compared to the northern and southern sections from seismicity behaviour, focal mechanisms and seismogenic depth. A temporal array of eight seismographs (including three broadband instruments) was augmented with three GeoNet stations bounding the array. This provided an average spacing of 14 km and a magnitude cut-off of ML 1.6 compared to the GeoNet national network cut-off of ML 2.6 and station spacing of 80-100 km. The Gutenberg-Richter distribution for the four month time frame analysed defned a b-value of 0.75 plus or minus 0.06 which may indicate a locked, heterogeneous zone under high-stress from fluid pressure or a predominance of thrust mechanisms over the survey period. Seismicity over the deployment was within the average range of the last 15 years. The 'horseshoe' shaped seismicity pattern observed from long-term national catologue data is similar for smaller magnitudes. While the central portion of the Alpine Fault is quieter with unusually low b-value, the region is not aseismic. Neither does it experience the level of microseismicity seen in creeping faults. The brittle-ductile transition varies laterally along the fault and is estimated at up to 15 km for most of the survey region but closer to 10 km for the region associated with the highest orogenic uplift rates which compares well with past studies. A local magnitude scale was developed from direct linear inversion of the pseudoWood-Anderson amplitudes and event-station distances. A linear inversion of data from the standard New Zealand magnitude equation characterised an attenuation parameter of 0.0167 km minus 1; more than double the value used in national local magnitude calculations (of 0.0067 km minus 1). Swarm clustering dominates the seismicity character of the time frame. Utilising the earthquake relocation program HypoDD, a selection of clusters both near the Alpine Fault and away from it resolve to point sources. Those close to the Alpine Fault are located in what may be the footwall of the Fault which may indicate that the velocity model has located the events too far to the northwest.</p>

Earthquake Swarm Analysis around Mt. Salak, West Java, Indonesia, Using BMKG Data from August 10 to November 24, 2019

Earthquake swarms commonly come approximately active tectonic and volcanic area. Interestingly, the swarm events occurred ~23 km southwest from Mt. Salak-Bogor, West Java, Indonesia, from August 10 to November 24, 2019, and were recorded by local/regional network of the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG). Our previous study showed that in this area a destructive ML 4.6 earthquake with thrust faulting occurred on September 8, 2012. The double-difference method was applied to update the hypocenter locations from the BMKG data. In the time period of ~3.5 months, we relocated 79 swarm events with ~9.4 km depth average for local magnitude (ML) 2.2 to 4.2. The source mechanism result for selected events shows a strike-slip faulting. Our interpretation is that these swarm events are probably related to stress change due to volcano-tectonic activity.

Konversi Empiris Summary Magnitude, Local Magnitude, Body-Wave Magnitude, Surface Magnitude, dan Moment Magnitude Menggunakan Data Gempabumi 1922-2020 di Nusa Tenggara Barat

The existence of magnitude type variation from existing earthquake catalogue sources show that uniforming process is necessary. Beside that these type of magnitude will saturates in certain value, which are different with moment magnitude (Mw) which is not saturated and can describe earthquake process better. Our research initially did compatibility test between summary magnitude which is largely used by BMKG with other magnitude type. Furthermore, the purpose of our research is determination of empirical relation between magnitude type summary magnitude (M), local magnitude (ML), body-wave magnitude (mb), dan surface magnitude (Ms) which are usually used by earthquake catalogues to Mw. Method used in this research is linear regression using data set from BMKG, ISC-EHB, USGS, and Global CMT catalogues with are limited in West Nusa Tenggara and surrounding area. Data used in this research contains of 24.703 earthquake events during period May 9th 1922 until June 27th 2020. The result of this research shows there was good relation between M magnitude type with others magnitude type. Our research also found a conversion formula of M, ML, MLv, mb, and Ms to Mw with well-defined correlation.

Orogenic Collapse and Stress Adjustments Revealed by an Intense Seismic Swarm Following the 2015 Gorkha Earthquake in Nepal

The April 25, 2015 Mw 7.9 Gorkha earthquake in Nepal was characterized by a peak slip of several meters and persisting aftershocks. We report here that, in addition, a dense seismic swarm initiated abruptly in August 2017 at the western edge of the afterslip region, below the high Himalchuli-Manaslu range culminating at 8156 m, a region seismically inactive during the past 35 years. Over 6500 events were recorded by the Nepal National Seismological Network with local magnitude ranging between 1.8 and 3.7 until November 2017. This swarm was reactivated between April and July 2018, with about 10 times less events than in 2017, and in 2019 with only sporadic events. The relocation of swarm earthquakes using proximal temporary stations ascertains a shallow depth of hypocenters between the surface and 20 km depth in the High Himalayan Crystalline slab. This swarm reveals an intriguing localized interplay between orogenic collapse and stress adjustments, involving possibly CO2-rich fluid migration, more likely post-seismic slip and seasonal enhancements.

Attenuation relationships of peak ground motions in the Jazan Region

In this study, attenuation relationships are proposed to more accurately predict ground motions in the southernmost part of the Arabian Shield in the Jazan Region of Saudi Arabia. A data set composed of 72 earthquakes, with normal to strike-slip focal mechanisms over a local magnitude range of 2.0–5.1 and a distance range of 5–200 km, was used to investigate the predictive attenuation relationship of the peak ground motion as a function of the hypocentral distance and local magnitude. To obtain the space parameters of the empirical relationships, non-linear regression was performed over a hypocentral distance range of 4–200 km. The means of 638 peak ground acceleration (PGA) and peak ground velocity (PGV) values calculated from the records of the horizontal components were used to derive the predictive relationships of the earthquake ground motions. The relationships accounted for the site-correlation coefficient but not for the earthquake source implications. The derived predictive attenuation relationships for PGV and PGA are$$ {\log}_{10}(PGV)=-1.05+0.65\cdotp {M}_L-0.66\cdotp {\log}_{10}(r)-0.04\cdotp r, $$ log 10 PGV = − 1.05 + 0.65 · M L − 0.66 · log 10 r − 0.04 · r , $$ {\log}_{10}(PGA)=-1.36+0.85\cdotp {M}_L-0.85\cdotp {\log}_{10}(r)-0.005\cdotp r, $$ log 10 PGA = − 1.36 + 0.85 · M L − 0.85 · log 10 r − 0.005 · r , respectively. These new relationships were compared to the grand-motion prediction equation published for western Saudi Arabia and indicate good agreement with the only data set of observed ground motions available for an ML 4.9 earthquake that occurred in 2014 in southwestern Saudi Arabia, implying that the developed relationship can be used to generate earthquake shaking maps within a few minutes of the event based on prior information on magnitudes and hypocentral distances taking into considerations the local site characteristics.